If you are making the egg-free version of this recipe, start by soaking the chia seeds in the water in a small bowl, leave them aside for 10 minutes or so. If you are going to make this recipe with an egg then you can skip this step
Whisk together the brown sugar, melted butter, vanilla extract and either the egg or soaked chia seeds.
Add the flour, cinnamon and rolled oats, and combine well (I swap to a wooden spoon at this stage).
Press two-thirds of the mix into a lined slice tin (I use a 20 x 20 cm tin, a cake tin will also work if you don't have a slice tin.
Lay the cooked apple slices on top of the base
Spoon the remaining oat mix on top of the apples, press down with a spatula (it doesn't matter if there are gaps)
Bake at 180 degrees Celsius (350 F) for 20-25 minutes or until the edges are golden and start to come away from the edge of the tin
Allow to cool
Slice into 12 pieces